My experience with running artemesias has been such that I no longer grow them after one brief unpleasant experience quite early in my gardening. Life is too short to spend it battling a plant that wants to take over the world. I think at this point you will find that it has sent roots under, around and through all of the plants it is near and it will take a lot of weeding and vigilance in looking for resprouts to get rid of it.

Silver mound artemesia IME stays put in a clump, though it will usually need a hard cutting back midseason since it tends to flop. Russian sage might be a good alternative for medium height silver foliage and has nice lavender flowers as well.

My apologies for inaccuracies. MO Botanical Garden, which is usually quite accurate IME, says it "spreads by underground rhizomes and may reach 3-6' wide if not restrained" which I misinterpreted based on my experience with other Artemisias. A slow spreader I guess, so not as hard to manage.

I've seen them get it wrong a couple of times...I bet they copied that text from another species of Artemesia.

I'd think that a harsh winter is likely to solve anyone's 'Powis Castle' "problem" for keeps, at least in zone 6 and above. Or if you want to up the odds that it dies, prune it severely in fall (typically, it's recommended that this operation be delayed until it's back in active growth in spring).

For a lot of people who haven't had much long-term success with 'Powis Castle' because of hardiness, commenting about its over-exuberance is like gardeners complaining that their roses have gotten too huge and floriferous.

Lol, going with Eric's 'suggestions'...a bit of extra water especially during the winter...will also probably take care of the problem.

Interesting to know about their zone issues. These made it through the harsh last winter with no issues. I might have to leave them be for the winter then and propagate them or try to move them in the spring in case some of them don't make it. Thanks for letting me know that there's no good way to cut them back.

I found that the trailing branches would root . Good plant in Texas. All the artemisias are. I think our soil and arid weather keep some of the bad ones in semi control. Silver mound on the other hand is too finicky. the heat and humidity turns it to nothingness. I am about ready to plant one of the bad ones. I have given it a hillside.

Again, 'Powys Castle' is not a "slow" runner--it doesn't run at all--it's more of a sub shrub, bushing out from a very small, woody footprint. Winter issues can be settled by giving it the best drainage possible.